The present invention relates to improvements in paper coating compositions for sizing paper. Surface sizing is the application of a non-pigmented coating to the surface of a paper web to seal up the sheet and improve the smoothness and strength of the surface of the paper for subsequent coating and/or printing.
The internal strength of paper is important for its intended use and for ease of manufacture. Increased internal bond strength is usually obtained by increasing the use of long length softwood pulp, a relatively expensive fiber, or by increased processing of the wood pulp by beating or refining the pulp to increase the fibrillation of the wood pulp which allows increased fiber to fiber bonding. This is an energy intensive step and depending on the amount of refining required, can be the rate determining step in the paper manufacturing process. Several chemical additives such as cationic starches have been used at the wet end of the paper machine for chemical enhancement of internal bond strength. It is difficult to retain 100% of these wet end chemical additives in the sheet and some are lost to the white water system resulting in poor chemical utilization.
Typical surface sizes comprise starches which can be used alone or in combination with hydrocolloids such as polyvinyl alcohol, carboxymethyl cellulose, wax emulsions and gums such as alginates. Preferred starches include those which have been chemically modified to prevent retrogradation following the cooking process. Suitable modified starches include those which are oxidized, substituted with cationic substituents, substituted with hydroxyalkyl ether substituents or are enzyme or acid thinned.
Winston, Jr. et al., U.S. Pat. No. 5,112,445 reports that a combination of gellan gum and starch demonstrates enhanced film formation on the surface of a coated paper sheet. The compositions disclosed by Winston, Jr. et al. comprise a hydroxyethyl starch ether in combination with a low-acyl gellan gum at weight ratios ranging from 80:1 to 160:1. Winston, Jr. does not teach starch/gellan gum ratios that enhance internal strength nor that affect size pickup levels. Nevertheless, there remains a desire in the art for improved surface sizing compositions providing improved properties.
The invention also relates to improvements in paper for use in ink jet printing. Ink jet printing is a non-impact digital printing process in which tiny droplets of ink are propelled from a nozzle to a paper surface without contact between the ink source and the surface. According to one type of ink jet printer, a piezoelectric element flexes one or more walls of a firing chamber which causes a pressure pulse and forces out a drop of ink. Some types of printers electrostatically charge the ink droplets and then use electric fields to deflect the droplets to a target spot on the paper or to a sump for recirculation.
Inks used for ink jet printing are frequently water based and are typically formulated to have lower viscosity than inks used for offset printing. Low viscosity is desired because less energy is required to pump and eject ink. One consequence of using low viscosity inks, however, is the undesirable tendency of the ink to "feather" (spread beyond the desired boundary) on the printed paper. The result is muddied undesirable printing.
The desire to better accommodate ink jet printing has led to the development of papers specifically formulated to reduce the tendency of ink jet ink to feather. One set of approaches has concentrated on the "coating side" of paper manufacture by modifying the surface sizes applied to the paper during manufacture. Some approaches use a size press to apply sizes comprising one or more of polyvinyl alcohol, starch, a surface sizing polymer emulsion such as Basoplast.RTM., a styrene-acrylic emulsion or a high surface area silica pigment to change ink absorptivity or openness of the sheet. The amount of such materials applied to the paper surface during use is referred to as "pickup" or "add on" and can be expressed in terms of pounds per unit area or pounds per ton of finished paper. These methods are expensive and/or difficult to control. Alternative methods on the coating side involve the use of a blade or roll or air knife coater and the application of more pigment and a binder such as styrene-butadiene latex but such methods are extremely expensive.
An alternative set of approaches to produce improved papers for ink jet printing has focused on modifications made on the "wet end" side of paper production and involves the selection of sizes and pigments added during paper formation. Wet end sizes ASA (alkenyl succinic anhydride), AKD (alkyl ketene dimer) and rosin are hydrophobic materials and are typically added in order to slow the speed of ink penetration into the paper matrix. The liquid permeability of paper is measured in the art according to the Hercules Size Test (HST) in which the time for an aqueous dye preparation to penetrate a paper sample is measured in seconds. In general the higher the test result the better with respect to the resistance of the paper to feathering. Nevertheless, papers characterized by very high HST results are characterized by such a low absorption of ink that the ink tends to smear when contacted. While wet end sizes have been used to increase the HST results of papers for ink jet printing their use alone has not sufficiently improved the quality of paper for ink jet printing. Accordingly, there remains a need for improved papers for ink jet printing.
Related to the use of AKD as a size for wet end modification of paper is the incorporation of mineral filler particles, commonly referred to as "ash" to the wet end during paper manufacture. These fillers can include a variety of minerals such as calcium carbonate, clay, and titanium dioxide. Fillers improve the opacity and brightness of the resulting paper and also allow for the reduction of cellulose fiber content of the paper thus reducing the raw material expense in paper production. Unfortunately, as more ash is added to the paper more AKD or similar size should be added to the paper to counteract the increase in liquid absorption caused by incorporation of the filler. In addition, more starch and/or more long fiber cellulose need to be added to the paper matrix to maintain strength. Nevertheless, there remains a desire in the art for methods to increase levels of ash incorporation while maintaining desirable strength and printing properties.